According to wave theory, wave means not only vibration but also propagation of the vibration. To be specific, 1. vibration can be decomposed into translational vibration, rotational vibration; 2. wave field divergence drives pressure wave, and wave field curl drives shear wave; 3. translational vibration is a combined action of divergence and curl, including not only pressure wave but also shear wave; 4. rotational vibration is related only to curl; 5. volume curl is complete curl, and surface curl is incomplete curl. Accordingly, a technology that can only detect translational vibration cannot completely separate pressure wave from shear wave. Only a technology that can detect volume curl or divergence can work out pure shear wave and pure pressure wave.
Spatial motion properties of wave include abundant information, which play important roles in aspects such as wave field separation, signal-to-noise ratio, fidelity, imaging precision, medium attribute analysis, or the like. However, an existing acquisition technology can only detect information such as amplitude, frequency, phase or the like, and could not detect the spatial properties of wave motions.
Currently, detection of seismic wave is realized by converting wave vibration into an electric signal (voltage, current) or then converting the electric signal into a digital signal. A method of converting mechanical motion into an electric signal applies nothing more than an electromagnetic detector, a capacitance detector, a piezoelectric detector and a fiber optic strain detector.
The electromagnetic detector and the capacitance detector are of line-vibration type and have working direction. Ideal direction filtering effect is cos θ, out(t)=A(t)cos θ. A(t) and θ are both unknown, only a single device cannot work out the true amplitude A(t) and the angle θ. An MEMS detector basically belongs to a capacitance detector, which also follows the above rule.
The fiber optic strain detector may be line-vibration or pressure-intensity type, and only a single device cannot determine vibration direction either. Accordingly, a single electromagnetic detector, a single capacitance detector or a single fiber optic strain detector cannot distinguish wave field vibration direction, even not having the function of detecting wave field divergence or curl.
The piezoelectric detector is of pressure-intensity type, and the output is related to pressure intensity of surrounding liquid medium, which is non-directional and cannot distinguish vibration direction. Pressure intensity in a liquid environment is isotropic and is equivalent to wave field divergence. But in a terrestrial solid environment, even if the detector is put in a liquid container, the detector still fails to realize divergence measurement.
A three component detector can work out wave field vibration direction θ and amplitude A(t) by a three-vector combination method. This is also the reason why the three component detector is called a vector detector, but it only measure translational vibration vector of a point, cannot detect nature, curl and divergence of vibration.
From the above, the existing seismic exploration technology cannot measure divergence and complete curl of seismic wave field, cannot solve problems of wave field vibration direction, propagation direction, true amplitude and separation of pressure and shear waves, thereby causing the existing seismic exploration technology cannot completely meet the resource exploration demand in aspects of formation imaging, formation lithology and fluid detection accuracy, embodied specifically in the following two aspects:
1) Restriction on Research and Analysis of Seismic Wave Property
Due to insufficient collection of information on seismic wave field, pressure wave and shear wave cannot be discomposed, such that demarcation between usable information and interference information is obscure. Analysis on an interference source is based on hypothesis and numerous tests, but the effect is unsatisfactory. For example, collection processing methods of improving fidelity on land and improving signal-to-noise ratio on mountain land are mutually restrictive, noise suppression effect needs to be further improved.
2) Restriction on Further Study on Wave Theory, Especially on Full-Elasticity Theory
Application of an elastic wave theory, through research, data collection, experiments and application of fully elastic wave equation, can obtain dielectric elastic parameters, accurately determine properties of the medium and spatial distribution state thereof, so as to improve lithology and fluid recognition accuracy. By spatial motion properties of seismic wave, e.g., media elastic parameters such as vibration property, divergence, curl etc., research on elastic wave is developed, which is technology developing direction for improving resource exploration accuracy. But, the existing seismic exploration technology cannot accurately measure divergence data and curl data of seismic wave field, thereby restricting the wave theory, especially further research on full-elasticity theory.
By the existing traditional seismic exploration technology, amplitude and arrival time of seismic wave field that are projected in vertical direction are recorded in the wild by utilizing a wave detector that vibrates vertically, which is called pressure wave exploration. In some circumstances, amplitude and arrival time of seismic wave field that are projected in horizontal direction are recorded by utilizing a wave detector that is placed horizontally, which is called converted wave exploration, and also called shear wave exploration. Such observation method has defects such as impure pressure wave and shear wave etc. The observed pressure wave and shear wave are separately and jointly subject to imaging process, and the pressure and shear waves are separately and jointly subject to attribute pickup, analysis and inversion. Because pressure and shear waves are mingled in the record of the observed seismic wave and are of mutual interference, signal-to-noise ratio of land pressure wave is low, shear wave cannot be recognized easily, seismic imaging accuracy is somewhat affected, lithology prediction and fluid prediction are of multiplicity of solutions.
For the above problem, there has not yet come up with an effective solution.